This invention relates to a system and method for determining the temperature and species make-up of a hot fluid, based on emitted thermal radiation.
Real-time temperature and chemical sensing methods are needed to monitor and control the efficiency, performance, and safety of combustion in systems such as turbine engines and industrial boilers. Strict control of temperature and exhaust composition is required to limit damage to sensitive components, control emissions, and increase output power and efficiency. Attempts to control combustion and achieve temperature uniformity have been hampered by the lack of sensors to monitor and spatially resolve the burner temperatures. In addition, there is a requirement to measure the chemical composition of the hot fluid to determine the extent to which combustion is complete, as un-combusted species may release additional heat when mixed with air.
For gas turbine engines, a critical limitation is the requirement that the combustor exit temperature stay below the damage threshold of the turbine blades. Accurate measurements of the spatial temperature distribution or pattern factor are required for experimental validation of burner capabilities and for use in active combustion control technologies. However, these measurements are rendered difficult by many factors, including the extreme conditions of the combustor environment (temperatures to 2500 K or 4000 F and pressures to 40 atmospheres), the inaccessibility of the measurement zone, and the possible presence of obscuring, surface-fouling soot.
Efforts underway to actively control the combustion process in gas turbines are severally limited by the lack of sensors capable of monitoring the effects of control actuators. In contrast to internal combustion engines that have real-time closed-loop control of fuel flow, current turbine engine technology depends on open-loop control of fuel air mixture, which results in large variations in conditions and efficiency across burners and among burners in the same engine.
Currently, thermocouples and pyrometers are used to infer the temperature of the hot fluid. Thermocouples are very useful at lower temperatures, but have performance, lifetime, and calibration issues at moderate temperatures (1,000-2000 K) and cannot be used at all at higher temperatures. Pyrometers are often used to optically measure the temperature of a hard surface within the boiler or turbine engine. Fluid temperature is then inferred from the surface temperature. Other optical techniques infer the combustion gas temperature from the intensity of the emitted radiation in one or more spectral regions characteristic of chemical intermediates in the combustion process. More sophisticated spectroscopic techniques such as tunable diode laser (TDL) absorption spectroscopy, laser-induced-fluorescence (LIF), coherent anti-Stokes Raman spectroscopy (CARS), and Fourier transform infrared (FTIR) spectroscopy have been used to determine temperature and composition in research applications. Such methods often have the disadvantage of requiring optical access from multiple viewpoints, critical alignment, and expert technical personnel to gather and interpret the data.
The invention offers significant advantages over these prior-art techniques. The invention solves these critical needs in one embodiment with a robust, flight-worthy sensor that can provide a variety of signals suitable for burner control, including temperature, fuel/air mixing ratio, and spatial distributions of these quantities.
The invention employs a passive measurement requiring a single port for optical access, has no critical alignment requirements, and is independent of pressure. It can be used over a large temperature range, and with no upper limit. It provides an accurate direct measurement of the temperature and/or relative concentrations of several molecular and particulate species within the hot fluid. Unlike methods based on inference, the measured component temperatures are independent of the details of the hot fluid source, and thus can be applied to any hot fluid containing those components. Spatially resolved measurements can be made using multiple views of the hot region. When compared to other optical methods, the invention is relatively simple, and the associated hardware is inherently less expensive, more reliable, and, ultimately, more readily adaptable as a flight-weight measurement system.
The preferred embodiment of the invention involves the use of passive optic probes, and a compact, flight-weight spectrograph to collect the bright structured emission features observable in the hot fluid flow path of a turbine engine. A simple optic probe is placed near the hot fluid, with a view of the hot region. The probe might have a diameter of several millimeters and consist of a collimating lens and an optical fiber, or a coherent fiber bundle. The fiber probe collects light emitted along a line-of-sight and directs it to the input of a spectrograph contained in a remote optical readout unit. The spectrograph disperses the light, and the dispersed light is recorded by an array detector, digitized, and processed to yield an emission spectrum. The visible and/or infrared emission spectra observed along one or more fields-of-view are continuously recorded at intervals of a fraction of a second and a spectral resolution of order 1/100th of the collected spectral interval. The recorded spectra are decomposed into component spectra of various emitting species in the hot fluid, such as water, soot, oxygen, carbon monoxide, and/or carbon dioxide. The temperature of each emitter can be determined from the shape of the spectra on a sub-second time scale, using spectral discrimination techniques. In contrast to pyrometric measurements based on the intensity of the emitted radiation or the relative intensity in multiple bands, this invention can determine the temperature based solely on the characteristic shape of a single species spectrum, which is independent of the amount of each species in the hot fluid. The spectra have sufficient information content to uniquely determine the temperature to better than 10xc2x0 C., and the relative concentration of the various gaseous emitters, and of soot, if present.
Spatially resolved measurements can be made using a variety of probe configurations. A probe may use a single fiber or a free space optical transfer system to interrogate a single field-of-view. Alternatively, a probe can use multiple optical fibers to define multiple fields-of-view terminating at the probe. The probe can be set back from the measurement region to define several near parallel fields of view though the hot region.
In an alternative embodiment, multiple fiber probes can be used to make simultaneous measurements of several views of the combustor. In a turbine engine, the circumferential pattern. of the temperature and/or composition can be determined using radial-viewing probes arrayed about the arc defined by the engine annulus.
In yet another alternative embodiment, multiple-fiber probes with overlapping views of the same region can be used to define volume elements within the flow. Spatial reconstruction techniques, such as tomography, can then be used to resolve the emitted radiation from each volume element, and determine the temperature and/or species concentration of each volume element.
The invention is suitable for collecting diagnostic information in engine tests, or can be used as a sensor as a part of an engine monitoring system, or an engine closed-loop control system. Data can be collected, processed, and output on a sub-second time scale. One or more output signals maybe generated for part of the control system. These include temperature; temperature spatial distribution; concentrations of soot, water, O2, and other component gases; spatial distribution of components; and mixing ratios of components, such as the water/oxygen ratio. The water/oxygen ratio is a direct indicator of the completeness of combustion, and can serve as a direct control signal for adjusting the fuel/air mixture in the combustor, in much the same way as an oxygen sensor is used to control an internal combustion engine.
The sensor may be placed in any desired and possible location in the combustion system to obtain information for diagnostic and control purposes. For example, the probe may be to positioned to measure the temperature or composition of the gas at the exit of the combustor and just prior to the inlet of the turbine, to monitor the temperature and latent heat content of the hot fluid to assure the safety of the turbine blade. Spatially resolved measurements may be made at this location to monitor and control the temperature distribution in the hot flow. Alternatively, the sensors may be placed upstream of the combustion zone with a view through the entire combustion flow field. This will allow the measurement of the maximum temperature, temperature distribution, and/or stoichiometry in the combusting gas, for use in controlling the fuel/air mixture.